Display panel unit and display device

ABSTRACT

In an embodiment, a display panel unit includes a reflective polarizing layer, a transparent layer, and a display panel. The polarizing layer transmits a first polarized component of light incident on an incident surface to an exit surface and reflects a second polarized component of the light. The transparent layer transmits the first and second polarized component of the light incident on a second surface to a first surface adhered to the incident surface and transmits the second polarized component reflected by the polarizing layer to be incident on the first surface to the second surface. The panel faces the exit surface, and selectively transmits the light exiting from the exit surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2013-217478, filed Oct. 18, 2013, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a display panel unitand a display device.

BACKGROUND

A display device having a prism sheet concentrating a spreading angle oflight from a light source upon a certain range and a reflectivepolarizer facing the prism sheet is used conventionally. When the lightpasses through the prism sheet, the reflective polarizer transmits, forexample, the p-polarized component of the light to a display panel (suchas a liquid crystal panel) side and reflects the s-polarized componentof the light onto a light source side.

The s-polarized component reflected by the reflective polarizer is againreflected by, for example, an optical element such as a reflective plateprovided on the light source side, and reaches back the reflectivepolarizer. Such a reflection process is repeated, and the s-polarizedcomponent converts into the p-polarized component by-and-by and passesthrough the reflective polarizer. When the reflective polarizer is used,the utilization efficiency of the light from the light source increasesand thus, the brightness of the display device increases.

In a display device with such a structure, interference fringes mayoccur between the prism sheet and the reflective polarizer. Theinterference fringe is one of the causes of display qualitydeterioration in the display device. Therefore, reduction or preventionof the occurrence of interference fringes is demanded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing the structure ofa liquid crystal display device of an embodiment.

FIG. 2 is a cross-sectional view showing the structure of a reflectivepolarizing plate of the embodiment.

FIG. 3 is a schematic diagram showing an example of how the liquidcrystal display device is used.

FIG. 4 is a table explaining structures of the liquid crystal displaydevice of the embodiment and their evaluation results.

FIG. 5 is a graph showing grades of interference fringes occurring inthe liquid crystal display device of the embodiment.

FIG. 6 is a table indicating some comparative examples used to avoidinterference fringes by methods different from the embodiment.

FIG. 7 is a cross-sectional view schematically showing the structure ofa conventionally-used liquid crystal display device.

FIG. 8 is a view schematically showing an example of interferencefringes appearing on a display surface of a liquid crystal displaydevice.

DETAILED DESCRIPTION

In general, according to one embodiment, a display panel unit includes areflective polarizing layer, a transparent layer, and a display panel.The reflective polarizing layer includes an incident surface of lightand an exit surface opposed to the incident surface, and the reflectivepolarizing layer transmits a first polarized component of the lightincident on the incident surface to the exit surface and reflects asecond polarized component of the light incident on the incident surfacewhich is orthogonal to the first polarized component. The transparentlayer includes a first surface adhered to the incident surface of thereflective polarizing layer and a second surface opposed to the firstsurface, and the transparent layer transmits the first polarizedcomponent and the second polarized component of the light incident onthe second surface to the first surface and transmits the secondpolarized component reflected by the reflective polarizing layer to beincident on the first surface to the second surface. The display panelfaces the exit surface of the reflective polarizing layer andselectively transmits the light exiting from the exit surface.

According to another embodiment, a display device includes a lightsource, a light guide, a reflective polarizing layer, a transparentlayer, a prism sheet, and a display panel. The light guide includes afirst incident surface on which light from the light source is incidentand a first exit surface from which the light incident on the firstincident surface exits. The reflective polarizing layer includes asecond incident surface on which the light from the first exit surfaceis incident and a second exit surface opposed to the second incidentsurface, and the reflective polarizing layer transmits a first polarizedcomponent of the light incident on the second incident surface to thesecond exit surface and reflects a second polarized component of thelight incident on the second incident surface which is orthogonal to thefirst polarized component. The transparent layer includes a firstsurface adhered to the second incident surface of the reflectivepolarizing layer and a second surface opposed to the first surface, andthe transparent layer transmits the first polarized component and thesecond polarized component of the light incident on the second surfaceto the first surface and transmits the second polarized componentreflected by the reflective polarizing layer to be incident on the firstsurface to the second surface. The prism sheet is interposed between thelight guide and the transparent layer, and the prism sheet includes aprism surface on which prism lenses are formed and which faces thesecond surface. The display panel faces the second exit surface of thereflective polarizing layer, and the display panel selectively transmitsthe light exiting from the second exit surface.

One of the embodiments will be explained with reference to theaccompanying drawings.

The present embodiment utilizes a transmissive type liquid crystaldisplay device as an example of a display device.

Initially, explained are the structure of a liquid crystal displaydevice used generally and interference fringes occurring in the liquidcrystal display device.

FIG. 7 is a cross-sectional view showing the schematic structure of agenerally-used liquid crystal display device 100. The liquid crystaldisplay device 100 includes a backlight unit 110 and a display panelunit 130. The dotted arrows in FIG. 7 show how a part of the light goeswithin the liquid crystal display device 100.

The backlight unit 110 includes a light source 111 which may be a linearlight source or may be composed of a plurality of spot light sourcesarranged on a single line, optical elements 120, light guide 112 whichgathers the light from the light source 111 and emits the uniform lighttoward the optical elements 120, and a reflective sheet 113 whichreflects the light going out of the lower part of the light guide 112 inFIG. 7 to turn the light back in the light guide 112.

The optical elements 120 include a diffusion sheet 121 which diffusesthe light from the light guide 112 and a first prism sheet 122 and asecond prism sheet 123 both of which concentrate the spreading angle ofthe light diffused by the diffusion sheet 121 to a certain angle range.The first prism sheet 122 and the second prism sheet 123 have thesurfaces to face the display panel unit 130. These surfaces are formedas prism surfaces on which a number of prism lenses each having auniform cross-sectional shape are extending in parallel. Note that thecross sectional shape of the prism lens may not be uniform.

The display panel unit 130 includes a reflective polarizing plate 131which transmits the p-polarized component of the light going out of theoptical elements 120 and reflects the s-polarized component of the lightorthogonal to the p-polarized component, a liquid crystal panel 132which selectively transmits the light going out of the reflectivepolarizing plate 131, and a polarizing plate 133 which transmits aspecific polarized component of the light which has passed through theliquid crystal panel 132. An air layer is formed between the secondprism sheet 123 and the reflective polarizing plate 131.

The s-polarized component reflected by the reflective polarizing plate131 is reflected by, for example, the prism surface of the second prismsheet 123 and the reflective sheet 113 and reaches the reflectivepolarizing plate 131 again. Through such repeated reflection processes,the s-polarized component by-and-by converts into the p-polarizedcomponent and passes through the reflective polarizing plate 131.

In FIG. 7, the optical elements 120 are partly warped. Such a warp iscaused by, for example, an external force applied to the liquid crystaldisplay device 100. The warp in the optical elements 120, especially,the warp of the second prism sheet 123 causes interference fringes onthe display surface of the liquid crystal display device 100.

FIG. 8 schematically shows interference fringes appearing on a displaysurface. Each of symbols d1 and d2 (d1<d2) in FIG. 8 indicates a pitchin a part of the interference fringes. In a part where the warp of thesecond prism sheet 123 is large, the pitch of the interference fringesbecomes narrow as d1, and in a part where the warp of the second prismsheet 123 is small, the pitch of the interference fringes becomes wideas d2.

It is considered that the interference fringes are caused due to theinterference between the light irradiated from the light source 111 toreach the second prism sheet 123 and the light reflected by thereflective polarizing plate 131 and again reflected by the reflectivesheet 113 or the like to reach the second prism sheet 123, or theinterference between the light and shade seen along the prism lens ofthe second prism sheet 123 and the reflective image generated by thereflective polarizing plate 131, or the like. A gap between the secondprism sheet 123 and the reflective polarizing plate 131 may becomenarrow locally when the gap becomes nonuniform. And this localnarrowness causes the above interferences.

Now, a liquid crystal display device of the present embodiment will beexplained.

FIG. 1 is a cross-sectional view schematically showing the structure ofliquid crystal display device 1 of the present embodiment. The liquidcrystal display device 1 includes a backlight unit 10 and a displaypanel unit 30. The dotted arrows in FIG. 1 show how a part of the lightgoes in the liquid crystal display device 1.

The backlight unit 10 includes a light source 11, a light guide 12, areflective sheet 13, and optical elements 20. The light source 11 is,for example, disposed at one end of the light guide 12 as a plurality ofspot light sources arranged linearly. The spot light sources are, forexample, light emitting diodes (LEDs) or organic electroluminescentdevices. The light source may be a linear light source formed of a coldcathode fluorescent tube or a hot cathode fluorescent tube instead.

The light guide 12 includes an incident surface 12 a which faces thelight source 11 and on which the light from the light source 11 isincident and an exit surface 12 b which faces the display panel unit 30and from which the light incident upon the incident surface 12 a exits.That is, the light guide 12 functions as, for example, a plane lightsource having a rectangular exit surface. The reflective sheet 13 isprovided on the lower surface of the light guide 12 in FIG. 1 (that is,provided on the surface of the light guide 12 opposite to the exitsurface 12 b side), and reflects the light going out of the lowersurface of the light guide 12 to turn back to the light guide 12.

The optical elements 20 include a diffusion sheet 21, a first prismsheet 22, and a second prism sheet 23 arranged in this order from thelight guide 12 side. The diffusion sheet 21, first prism sheet 22, andsecond prism sheet 23 are all shaped substantially the same as the exitsurface 12 b of the light guide 12 when viewed in a plane view anddisposed on the exit surface 12 b in this order.

The diffusion sheet 21 diffuses the light from the exit surface 12 b ofthe light guide 12 and uniforms the brightness of the light incidentupon the first prism sheet 22. The diffusion sheet 21 may include, forexample, a light-diffusing structure on its surface which diffuses thelight, or may include microparticles or the like to have a refractiveindex different from that of the main structural material. Each of thefirst prism sheet 22 and the second prism sheet 23 has a prism surfaceon which a plurality of prism lenses each having a uniformcross-sectional shape are extending in parallel. The cross sectionalshape of the prism lens is, for example, a triangle whose vertex angleis substantially 90 degrees. The first prism sheet 22 and the secondprism sheet 23 are layered such that the ridgelines of the prism lensesare orthogonal to each other, for example. Note that the cross sectionalshape of the prism lens of the first prism sheet 22 and the second prismsheet 23 may not be uniform.

Using the work of prism lens, the first prism sheet 22 and the secondprism sheet 23 concentrate the spreading angle of the light diffused bythe diffusion sheet 21 upon a certain angle range. The ends of the lightsource 11, the light guide 12, the reflective sheet 13, the diffusionsheet 21, the first prism sheet 22, and the second prism sheet 23 areheld by, for example, a frame-shaped bezel.

The display panel unit 30 faces the exit surface 12 b of the light guide12. Furthermore, the ends of the display panel unit 30 are supported bya frame or a bezel. The display panel unit 30 includes a reflectivepolarizing plate 31 as a reflective polarizer, a liquid crystal panel 32as a display panel, and a polarizing plate 33.

The liquid crystal panel 32 includes, for example, an array substrate 32a with a large number of pixel electrodes, a counter substrate 32 bfacing the array substrate 32 a interposing a microgap therebetween, anda liquid crystal layer composed of liquid crystal components sealedbetween the array substrate 32 a and the counter substrate 32 b. Thecounter substrate 32 b includes color filters corresponding to each ofthe colors red, green, and blue, or the like.

The reflective polarizing plate 31 is adhered to the rear surface of theliquid crystal panel 32, that is, one of the surfaces of the arraysubstrate 32 a which faces the backlight unit 10. The polarizing plate33 is adhered to the display surface of the liquid crystal panel 32,that is, one of the surfaces of the counter substrate 32 b which isopposite to the surface facing the array substrate 32 a. The reflectivepolarizing plate 31 and the polarizing plate 33 face the display area ofthe liquid crystal panel 32.

The liquid crystal panel 32 selectively applies voltage to each of thepixel electrodes to switch liquid crystal molecules in the liquidcrystal layer for controlling the transmission(transmissive/nontransmissive) of the light exiting the reflectivepolarizing plate 31 on a pixel-by-pixel manner in the display area. Theunnecessary polarized light component passing through the liquid crystalpanel 32 is removed by the polarizing plate 33, and a user can recognizedisplay images on the display area.

The reflective polarizing plate 31 includes a transparent layer 300 anda reflective polarizing layer 310 arranged in this order from thebacklight unit 10 side. The reflective polarizing layer 310 has anincident surface 310 a on which the light from the exit surface 12 b ofthe light guide 12 is incident, and an exit surface 310 b opposed to theincident surface 310 a. The transparent layer 300 is adhered closely tothe incident surface 310 a of the reflective polarizing layer 310.

The reflective polarizing layer 310 passes a first polarized componentof the light incident on the incident surface 310 a through the exitsurface 310 b and reflects back a second polarized component of thelight incident on the incident surface 310 a to the backlight unit 10side. The transparent layer 300 has a first surface 300 a adheredclosely to the incident surface 310 a of the reflective polarizing layer310 and a second surface 300 b opposed to the first surface 300 a. Thetransparent layer 300 passes the first polarized component and thesecond polarized component of the light coming from the backlight unit10 and incident on the second surface 300 b through the first surface300 a and passes the light coming from the reflective polarizing layer310 and incident on the first surface 300 a (for example, the lighthaving the second polarized component reflected by the reflectivepolarizing layer 310 and incident on the first surface 300 a) throughthe second surface 300 b. In the present embodiment, the first polarizedcomponent is given the p-polarized component and the second polarizedcomponent is given the s-polarized component which is orthogonal to thep-polarized component. Note that no limitation is intended by the above,that is, the reflective polarizing plate 31 or the liquid crystaldisplay device 1 may be structured given that the first polarizedcomponent is the s-polarized component and the second polarizedcomponent is the p-polarized component.

With reference to the cross-sectional view of FIG. 2, the structure ofthe reflective polarizing plate 31 is explained in detail. Thetransparent layer 300 includes a hardcoat (HC) layer 301, a transparentbase material 302, and a transparent adhesive layer 303 arranged in thisorder from the backlight unit 10 side. The reflective polarizing layer310 includes a reflective layer 311, a transparent adhesive layer 312,and a polarizing layer 313 arranged in this order from the backlightunit 10 side.

In the example of FIG. 2, the incident surface 310 a of the reflectivepolarizing layer 310 corresponds to one of the surfaces of thereflective layer 311 which faces the transparent layer 300, and the exitsurface 310 b corresponds to one of the surfaces of the polarizing layer313 which faces the display panel unit 30. Furthermore, the firstsurface 300 a of the transparent layer 300 corresponds to one of thesurfaces of the transparent adhesive layer 303 which faces thereflective polarizing layer 310, and the second surface 300 bcorresponds to one of the surfaces of the HC layer 301 which faces thesecond prism sheet 23.

Each structural element of the transparent layer 300 is formed of, forexample, a material which suppresses an optical loss and an opticalphase difference between the elements as low as possible. Specifically,the optical loss of the transparent layer 300 should preferably belowered to such an extent that the light transparency exceeds 95%.Furthermore, substantially no optical phase difference should preferablybe found within the transparent layer 300, that is, substantially nooptical phase difference should preferably be found between the lightbefore passing through the transparent layer 300 and the light afterpassing through the transparent layer 300.

The HC layer 301 protects the surface of the transparent base material302 in case of, for example, contacting the prism lens of the secondprism sheet 23. The HC layer 301 is formed by applying, for example,acrylic resin to the surface of the transparent base material 302. Thematerial of the transparent base material 302 is formed of, for example,polyethylene terephthalate (PET), triacetylcellulose (TAC),cycloolefinpolymer (COP), or polymethyl methacrylate (PMMA) as acrylicresin. The transparent adhesive layer 303 is formed by bonding thetransparent base material 302 to the incident surface 310 a. Thetransparent adhesive layer 303 is, for example, an adhesive material ofacrylic resin.

The reflective layer 311 passes the p-polarized component of the lightincident on the incident surface 310 a to the polarizing layer 313 sideand reflects the s-polarized component to the second prism sheet 23side. The transparent adhesive layer 312 is formed by bonding thereflective layer 311 to the polarizing layer 313. The transparentadhesive layer 312 does not substantially block the transmission of anypolarized component of the light. The polarizing layer 313 passes thep-polarized component of the light which has passed through thereflective layer 311 and the transparent adhesive layer 312 to the exitsurface 310 b side and absorbs the other polarizing components. Thes-polarized component reflected by the reflective layer 311 again headsfor the backlight 10 side and is again reflected by, for example, theprism surface of the second prism sheet 23 or the reflective sheet 13.The s-polarized component then reaches the reflective polarizing plate31 again. The s-polarized component is reflected repeatedly between thereflective polarizing plate 31 and the backlight 10 and is soonconverted into the p-polarized component, and eventually, passes throughthe reflective polarizing layer 310. As can be understood from theabove, when the s-polarized component and the p-polarized componentcontained in the light from the backlight unit 10 are used effectively,the brightness of the liquid crystal display device 1 can be improved.

The backlight unit 10 and the display panel unit 30 are assembled byfixing their ends with, for example, an adhesive material, a bondingmaterial, or a spacer, or by fixing their ends by a bezel. For example,the assembled liquid crystal display device 1 is used in such a statethat it is installed in a casing 201 of a smartphone 200 as in FIG. 3.The liquid crystal display device 1 can be used, instead in such asmartphone, as a display of a tablet computer, a mobile phone, anotebook type personal computer, an electronic dictionary, or a variouselectronic or audio visual devices such as a television device.

Here, as shown in FIG. 2 by a chain double-dashed line, a planeincluding the ridgelines of the prism lenses formed on the second prismsheet 23 is defined as a prism surface 23 a. The prism surface 23 a is asurface facing the second surface 300 b of the transparent layer 300.Furthermore, the gap between the incident surface 310 a of thereflective polarizing layer 310 and the prism surface 23 a is defined asG, and the thickness of the transparent layer 300 is defined as H.

Here, since the air layer is formed between the transparent layer 300and the prism surface 23 a, H≦G. Furthermore, the thickness of the airlayer, that is, a gap between the prism surface 23 a and the secondsurface 300 b of the transparent layer 300 should preferably be lessthan the thickness of the transparent layer 300.

The gap G may be narrowed locally by warpage caused by an external forceapplied to the second prism sheet 23 or the reflective polarizing plate31, or warpage or curving of the second prism sheet 23 or the reflectivepolarizing plate 31 themselves. If the gap G is locally narrowed beyonda certain extent, interference fringes explained with reference to FIG.8 will occur. However, the reflective polarizing plate 31 of the presentembodiment includes the transparent layer 300 between the reflectivepolarizing layer 310 and the second prism sheet 23. Therefore, even ifthe second prism sheet 23 or the like is warped by, for example, anexternal force, the gap G can be kept greater than the thickness H.Accordingly, the occurrence of interference fringes can be reduced orprevented.

[Examples]

in the following examples, the structure and function of the liquidcrystal display device 1 are explained in detail. Note that the scope ofthe invention is not limited to the following disclosure of theseexamples.

Liquid crystal display devices 1 with transparent layers 300 havingvarying thickness H were prepared for visual evaluation of grades ofinterference fringes. Results were obtained as indicated in the table ofFIG. 4. The targets of the visual evaluation were five liquid crystaldisplay devices 1 with the transparent layers 300 having a thickness of(1) none (H=0 μm); (2) H=10 μm; (3) H=25 μm; (4) H=37 μm; and (5) H=59μm. In each device of (1) to (5), the first prism sheet 22 was preparedas a prism sheet composed of a polyester base material and acrylic resinprism lenses with 24 μm pitches, with a smoothed rear surface of thebase material and a total thickness of 65 μm. Furthermore, in eachdevice of (1) to (5), the second prism sheet 23 was prepared as a prismsheet composed of a polyester base material and acrylic resin prismlenses with a pitch of 24 μm, with a rear surface of the base materialprovided with a mat layer and a total thickness of 70 μm. The gap G wasthe same in each device of (1) to (5).

In the device of (2), the transparent layer 300 was prepared as a filmincluding a transparent base material 302 formed of PET and having athickness of 5 μm, and a transparent adhesive layer 303 (glue) formed ofacrylic resin and having a thickness of 5 μm. In the device of (2), thetransparent layer 300 does not include HC layer 301.

In the device of (3), the transparent layer 300 was prepared as a filmincluding a transparent base material 302 formed of PMMA and having athickness of 20 μm, and a transparent adhesive layer 303 (glue) formedof acrylic resin and having a thickness of 5 μm. In the device of (3),the transparent layer 300 does not include HC layer 301.

In the device of (4), the transparent layer 300 was prepared as a filmincluding a HC layer 301 formed of acrylic resin and having a thicknessof 7 μm, a transparent base material 302 formed of TAC and having athickness of 25 μm, and a transparent adhesive layer 303 (glue) formedof acrylic resin and having a thickness of 5 μm.

In the device of (5), the transparent layer 300 was prepared as a filmincluding a HC layer 301 formed of acrylic resin and having a thicknessof 7 μm, a transparent base material 302 formed of TAC and having athickness of 40 μm, and a transparent adhesive layer 303 (glue) formedof acrylic resin and having a thickness of 12 μm.

In the table of FIG. 4, the item “transparency” indicates thetransparency of the transparent layer 300. The device of (1) lacks thetransparent layer 300 and thus, the transparency is 100%. The devices of(2) to (5) exert substantially 100% transparency.

In the table of FIG. 4, the item “N/R” indicates the result of thevisual evaluation of interference fringes generated by applying anexternal force to the liquid crystal display device 1 of each of (1) to(5). In (1) to (3), the occurrence of interference fringes was not fullyprevented (results were NG). On the other hand, in (4) and (5),interference fringes were almost unrecognizable and an excellentpreventive effect was obtained (results were OK).

The interference fringe state was graded from 1 (bad) to 5 (light) andgrades below 4 were evaluated “NG” and grades 4 to 5 were evaluated“OK”. FIG. 5 is a graph showing a relationship between the interferencefringe grades and the thickness H of the transparent layer 300. Thesampling points used for the graph include the thickness H of each of(1) to (5) shown in FIG. 4. The curve in the graph is an approximatecurve going along grade evaluated at each of the sampling points. Theinterference fringe grade evaluated when the thickness H=0 μm was setto 1. The interference fringe grade gradually improved when thethickness H approached 10 μm showing a steeple rise, exceeded grade 4when the thickness H approached 30 μm, and then almost reached grade 5.

The graph shows that interference fringes are suppressed better as thethickness H increases. Furthermore, around a thickness of 30 μm, theinterference fringe grade exceeds 4 which is a given criterion for theevaluation. The above proves that the present embodiment with thetransparent layer 300 can achieve at least reduction or prevention ofthe occurrence of interference fringes, and can achieve a better resultin suppression of interference fringes if the thickness H of thetransparent layer 300 is approximately 30 μm or more.

If the thickness H is near 50 μm, the interference fringe grade reaches5 which is the best condition in this evaluation. This proves that thepresent embodiment can achieve an excellent result in suppression ofinterference fringes if the thickness H of the transparent layer 300 isapproximately 50 μm or more.

[Comparison with Other Methods]

Now, the structure of the present embodiment is compared with the othermethods used for reduction or prevention of interference fringes withreference to FIG. 6.

As the other methods, the following (I) to (IV) were adopted.

(I) In this method, the liquid crystal display device 1 was preparedwithout a transparent layer 300 but with a diffusion material added tothe transparent adhesive layer 312 of the reflective polarizing layer310. The transparent adhesive layer 312 with the diffusion materialadded thereto diffuses the light and shade of interference fringes, andthe visibility of the interference fringes decreases.

(II) In this method, the liquid crystal display device 1 was preparedwithout a transparent layer 300 but with the polarizing plate 33 whosesurface has been subjected to a diffusion treatment (for example, anantiglare treatment) to be a diffusion layer. The diffusion layerdiffuses the light and shade of interference fringes, and the visibilityof the interference fringes decreases.

(III) In this method, the liquid crystal display device 1 was preparedwithout a transparent layer 300 but with the reflective polarizing layer310 whose surface at the second prism sheet 23 side (that is, incidentsurface 310 a) has been subjected to a diffusion treatment (for example,an antiglare treatment) to be a diffusion layer. The diffusion layerdiffuses the light passing through and reflected upon the reflectivelayer 311, and the occurrence of interference fringe itself can beprevented.

(IV) In this method, the liquid crystal display device 1 was preparedwithout a transparent layer 300 but with a diffusion sheet insertedbetween the reflective polarizing layer 310 and the second prism sheet23. The diffusion sheet diffuses the light from the second prism sheet23 and incident on the reflective polarizing layer 310 and the lightreflected by the reflective polarizing layer 310 and incident on thesecond prism sheet 23, and the occurrence of interference fringe itselfcan be prevented.

Liquid crystal display devices of the methods (I) to (IV) and the liquidcrystal display device 1 of the present embodiment with the transparentlayer 300 on the incident surface 310 a of the reflective polarizinglayer 310 were prepared for evaluating the avoidance of occurrence ofinterference fringes and the change ratio of display brightness. Theliquid crystal display devices were all prepared the same except theparts explained in (I) to (IV) above. The brightness of a liquid crystaldisplay device which was prepared not based on any method of the presentembodiment or of above (I) to (IV), that is, the brightness of a liquidcrystal display device shown in FIG. 1 excluding the transparent layer300 was used as the criterion for the evaluation of the change ratio ofthe brightness.

The evaluation showed that the liquid crystal display devices of themethods (I) and (II) slightly reduced the interference fringes. However,the change ratio of the brightness was −6% or less in the liquid crystaldisplay device of the method (I) and the change ratio of the brightnesswas −5% or less in the liquid crystal display device of the method (II).

Furthermore, the evaluation showed that the liquid crystal displaydevices of the methods (III) and (IV) almost completely avoided theoccurrence of the interference fringes. However, the change ratio of thebrightness was −5% or less in the liquid crystal display device of themethod (III) and the change ratio of the brightness was −10% or less inthe liquid crystal display device of the method (IV).

The liquid crystal display device 1 of the method of the presentembodiment can perform substantially complete avoidance of interferencefringes particularly when the thickness H is 30 μm or more. Or, thepresent embodiment can reduce the visibility of interference fringesremarkably. In the present embodiment, the transparent layer 300 isinterposed between the prism surface 23 a and the reflective polarizinglayer 310 and is adhered closely to the reflective layer 311. Such astructure always keeps the gap G greater than the thickness H of thetransparent layer 300 even if the gap G is reduced by warpage or thelike of the prism surface 23 a. As shown in FIG. 8, the interferencefringes appearing on the display area become larger as the gap Gdecreases; however, the gap G never becomes less than the thickness H ofthe transparent layer 300 in the present embodiment and thus, the gap Gnever becomes excessively narrow in the present embodiment. Furthermore,as can be understood from the above examples, the interference fringesbecome recognizable when the gap G is less than 30 μm, and thus, thethickness H of the transparent layer 300 is 30 μm or more to maintain agap G of at least 30 μm.

Consequently, the interference fringes do not occur at all, or occurvery limitedly. Thus, the visibility of interference fringes can bereduced remarkably.

Furthermore, the change ratio of the brightness is substantially 0% (nochange in the brightness) in the present embodiment. That is, the liquidcrystal display device 1 of the present embodiment can reduce or preventthe occurrence of interference fringes without affecting the brightness.

The structure of the above embodiment may be changed into various forms.

For example, in the present embodiment, the transmissive type liquidcrystal display device 1 is disclosed as an example of the displaydevice including the reflective polarizing plate 31. However, thereflective polarizing plate 31 can be used as an optical element in anyother display devices such as a reflective type liquid crystal displaydevice, a transreflective type liquid crystal display device, or anon-liquid crystal display device.

The structure of the reflective polarizing layer 310 is not limited tothe disclosure of the above embodiment. The reflective polarizing layer310 may be structured as a circular polarizing separation type layer ora wire grid type layer.

The structure of the transparent layer 300 is not limited to thedisclosure of the above embodiment. For example, the transparent layer300 may not include the HC layer 301. The advantage of the reduction orprevention of the occurrence of interference fringes mainly depends onthe thickness H of the transparent layer 300. Therefore, the transparentlayer 300 without the HC layer 301 can reduce or prevent the occurrenceof interference fringes.

The optical elements of the liquid crystal display device 1 are notlimited to those shown in FIG. 1. The liquid crystal display device 1may include additional optical elements in addition to the opticalelements shown in FIG. 1 or may exclude part of the optical elementstherein.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A display device comprising: a light source; alight guide including a first incident surface at which light from thelight source enters and a first exit surface from which the lightincident on the first incident surface exits; a prism sheet over thelight guide; a transparent layer over the prism sheet; a reflectivepolarizing layer over the transparent layer, the reflective polarizinglayer including a second incident surface at which the light from thefirst exit surface enters and a second exit surface opposed to thesecond incident surface, the reflective polarizing layer configured totransmit a first polarized component of the light incident on the secondincident surface to the second exit surface and to reflect a secondpolarized component of the light incident on the second incident surfaceto the transparent layer, the second polarized component beingorthogonal to the first polarized component; and a display panel overthe reflective polarizing layer, the display panel configured toselectively transmit the light exiting from the reflective polarizinglayer, wherein: the transparent layer includes a first surface adheredto the second incident surface of the reflective polarizing layer and asecond surface opposed to the first surface, the transparent layer beingconfigured to transmit the first and second polarized componentsincident on the second surface to the first surface and to transmit thesecond polarized component incident on the first surface by beingreflected by the reflective polarizing layer to the second surface, thetransparent layer includes a transparent base and a first transparentadhesive layer configured to attach the transparent base to the secondincident surface, and the transparent base includes a film consistingessentially of a single transparent material and transmits the lightwith substantially no optical phase difference.
 2. The display device ofclaim 1, wherein a light transparency of the transparent layer is 95% ormore.
 3. The display device of claim 1, wherein a thickness of thetransparent layer is 30 μm or more.
 4. The display device of claim 1,wherein a thickness of the transparent layer is 50 μm or more.
 5. Thedisplay device of claim 1, wherein the reflective polarizing layerincludes a reflective layer configured to reflect the second polarizedcomponent of the light incident on the second incident surface, apolarizing layer configured to transmit the first polarized component ofthe light passing through the reflective layer and to absorb otherpolarized components, and a second transparent adhesive layer configuredto attach the reflective layer and the polarizing layer to each other.6. An optical sheet disposed between a display panel and a prism sheet,the optical sheet comprising: a transparent layer; and a reflectivepolarizing layer over the transparent layer, the reflective polarizinglayer including an incident surface at which the light from thetransparent layer enters and an exit surface opposed to the incidentsurface, the reflective polarizing layer configured to transmit a firstpolarized component of the light incident on the incident surface to theexit surface and to reflect a second polarized component of the lightincident on the incident surface to the transparent layer, the secondpolarized component being orthogonal to the first polarized component,wherein the transparent layer includes a first surface adhered to theincident surface of the reflective polarizing layer and a second surfaceopposed to the first surface, the transparent layer being configured totransmit the first and second polarized components incident on thesecond surface to the first surface and to transmit the second polarizedcomponent incident on the first surface by being reflected by thereflective polarizing layer to the second surface, the transparent layerincludes a transparent base and a first transparent adhesive layerconfigured to attach the transparent base to the incident surface, thetransparent base includes a film consisting essentially of a singletransparent material and transmits the light with substantially nooptical phase difference, and the optical sheet is disposed between adisplay panel and a prism sheet, such that the transparent layer is overthe prism sheet.
 7. The optical sheet of claim 6, wherein a lighttransparency of the transparent layer is 95% or more.
 8. The opticalsheet of claim 6, wherein a thickness of the transparent layer is 30 μmor more.
 9. The optical sheet of claim 6, wherein a thickness of thetransparent layer is 50 μm or more.
 10. The optical sheet of claim 6,wherein the reflective polarizing layer includes a reflective layerconfigured to reflect the second polarized component of the lightincident on the incident surface, a polarizing layer configured totransmit the first polarized component of the light passing through thereflective layer and to absorb other polarized components, and a secondtransparent adhesive layer configured to attach the reflective layer andthe polarizing layer to each other.